Field of the Invention
The present invention relates to a powder magnetic core, and more particularly, to a powder magnetic core which is designed to have low initial permeability so that the core may show high permeability when applied with high-intensity magnetic field and as a consequence may exhibit excellent direct-current bias characteristics.
Prior Art
Powder magnetic cores can be produced with high yield even if articles to be produced are small in size or complicated in shape, and thus have come to be widely used in place of conventionally popular laminated magnetic cores using silicon steel sheets.
Generally, a powder magnetic core is produced in the manner described below.
First, a soft magnetic alloy having a predetermined composition is subjected to mechanical crushing or atomizing to obtain powder (soft magnetic powder) with a predetermined particle size distribution.
Subsequently, the soft magnetic powder is admixed uniformly with predetermined amounts of an insulating material and a binder component so that the powder magnetic core to be obtained may have high electrical resistivity. The insulating material used in this case is, for example, oxide powder such as Al2O3 powder or SiO2 powder, or nitride powder such as powder of AlN, Si3N4, or BN. As the binder component, water glass having electrical insulating properties or organic polymer such as silicone resin is used.
In the following description, the insulating material and the binder component are collectively referred to as the xe2x80x9cinsulating binderxe2x80x9d.
The mixture obtained in this manner is filled in a mold and compacted under a predetermined pressure, to obtain a green compact of powder magnetic core. At this time, in order to enhance the compactibility, the mixture is usually mixed with a predetermined amount of lubricant such as zinc stearate.
Lastly, the green compact is heat-treated to release the strain accumulated therein during the compaction, thereby obtaining a target powder magnetic core.
The powder magnetic core produced in this manner generally shows a magnetization curve (B-H curve) such that the magnetic induction gradually increases with increasing strength of a direct-current magnetic field (applied magnetic field) and is saturated when applied with a certain strength of magnetic field. Permeability (differential relative permeability) at a certain strength of direct-current magnetic field in the process of such gradual increase of the magnetic induction is defined as a value which is obtained by dividing a variation of the magnetic induction, observed when the magnetic field is slightly changed with a low-intensity alternating-current magnetic field superimposed on the direct-current magnetic field, by the slight change of the magnetic field. Accordingly, as the gradient of the B-H curve lessens, that is, as the strength of the applied magnetic field increases, the differential relative permeability decreases and thus the permeability lowers. After saturation magnetization is reached, the permeability virtually equals xe2x80x9c1xe2x80x9d.
In the case of a high-permeability powder magnetic core produced using soft magnetic powder such as sendust powder as a material, if the powder magnetic core is used with a heavy current passed, an intense direct-current magnetic field is applied to the core, so that the magnetic induction of the core rapidly approaches saturated state. As a result, a problem arises in that the permeability decreases toward xe2x80x9c1xe2x80x9d. Namely, this type of high-permeability powder magnetic core is poor in direct-current bias characteristics.
Generally, powder magnetic cores with an initial permeability of about 60 to 125 are put to practical use in various industrial fields. In the case of such powder magnetic cores, if a high-intensity magnetic field of, for example, 16 kA/m or above is applied, the permeability becomes extremely low, so that the cores cannot be put to actual use.
Thus, an effective measure to ensure the required level of permeability while at the same time suppress deterioration in the direct-current bias characteristics even with the application of a high-intensity magnetic field of 16 kA/m or above, for example, is to lower the initial permeability of a powder magnetic core to be produced.
It is generally known that the permeability of a powder magnetic core is a function of the density of the core. Accordingly, in view of the fact that a powder magnetic core with low density shows low permeability, it can be said that the initial permeability of a powder magnetic core can be effectively lowered by reducing the density of the core.
The following requirements should, however, be taken into consideration: The powder magnetic core should have magnetic characteristics such that the magnetic induction thereof increases with increasing strength of the applied magnetic field and that saturation magnetization is finally reached. The powder magnetic core, even though the initial permeability thereof is low, should have a saturation magnetic induction satisfying the required level for actual use. The powder magnetic core should be able to be manufactured with an industrially acceptable high yield.
The present invention was made in view of the above requirements, and an object thereof is to provide a novel powder magnetic core of which the reduction in permeability is small even when applied with high-intensity magnetic field and which can therefore be used practically with an applied magnetic field of up to high intensity.
To achieve the above object, the present invention provides a powder magnetic core made from powder containing an Fe-based soft magnetic alloy as a major component, wherein, provided that an initial permeability of the powder magnetic core is xcexc0 and that a permeability of the powder magnetic core observed when a magnetic field of 24 kA/m is applied to the powder magnetic core is xcexc, xcexc0 and xcexc fulfill a relationship of xcexc/xcexc0xe2x89xa70.5. More particularly, there is provided a powder magnetic core which is made from 60 to 75 volume % soft magnetic powder having an aspect ratio of 1 to 1.5 and the balance containing an insulating binder as a major component, wherein the content of the insulating binder is 5 to 20 parts by weight with respect to 100 parts by weight of the soft magnetic powder.